1. Field of the Invention
The invention relates to tip party identification in a two party line for a telephone communication switching system, and more particularly to detect the presence of a specific current to ground when one of the two subscribers is connected.
2. Description of the Prior Art
The invention was developed for the system shown in U.S. Pat. No. 3,767,863, issued Oct. 23, 1973, by Borbas et al for a Communication Switching System with Modular Organization and Bus, hereinafter referred to as the System S2 patent.
In previous systems the side of line (SOL) signal was detected using a differential current sensing relay. One such system using a differential current sensing relay is shown in U.S. Pat. No. 3,678,197, issued July 18, 1972 by Panter et al for a Dial Pulse Incoming Trunk and Register Arrangement. This relay is fairly large and does not readily lend itself to card mounting. Also this relay has an inherent problem with sensitivity to shock, vibration, and orientation. Further, in past years this current sensing relay circuit has required extensive maintenance, due to the extremely short travel of the relay springs and the accuracy with which this adjustment must be maintained.
Two applications of operational amplifiers in telephone circuits are Tjaden, U.S. Pat. No. 3,622,709 for a Supervisory Circuit for Telephone Lines and Stewart, U.S. Pat. No. 3,812,303 for a Differential Loop Current Detector. Tjaden shows an operational amplifier with voltage detectors which is utilized to indicate the opening and closing of the two wire loop to indicate seizure and dial pulsing. This system performs a supervisory function for detecting the operative state of circuits only. Stewart is directed more closely to the subject of the present invention as it actually does sense and measure direct current unbalances in a two wire loop. However, it is designed with two operational amplifiers and may be intended to be used as an instrument to actually measure the amount of unbalance. This measurement could be made in conjunction with a meter or any voltage measuring device to measure the absolute value of this current unbalance. In order to be used as replacement for an existing SOL detector relay which senses tip party identification, additional components would have to be added to the circuit. Even on adding the additional components it still could have limitations because of the large number of balanced or matched resistors required which increases the cost and reliability of the circuit. Further, the resistors would also have to be thermally tracking in that as the temperature changes, the variation in resistance from resistor to resistor should be similar (percentage). Also it would appear that the circuit would not reject leakage and would be sensitive to 60 hz noise which would produce false outputs.